Thromboembolic obstruction to three major components of the circulation--arterial, venous and intravascular foreign surfaces--contributes to premature death and disability in Western society. In many, but not all, of these conditions associated with thromboembolism, heparin and warfarin are the drugs of choice. It is the purpose of this presentation to provide some common ground in the area of anticoagulant prophylaxis that will be of intrinsic value for decision making in cardiac, cerebral and peripheral vascular disease. Only those aspects of the hemostatic mechanism most relevant to the antithrombotic action of heparin and warfarin are discussed. Assays for both drugs as well as some practical guidelines for their use in low, medium and high dose regimens are outlined. Techniques for improving the benefit/risk ratio for each drug are specifically detailed.